I just came across the same problem while working on a Windows Store App. After several problems I figured out that the easiest why to define such a collection in XAML is to simply create a subclass:

namespace my.name.space {
    public class ObservableMyObjectCollection: ObservableCollection<MyObject> {
    }
}

And then use it like so

<local:ObservableMyObjectCollection
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:my.name.space">

   <MenuItem Command="ApplicationCommands.Cut"/>
   <MenuItem Command="ApplicationCommands.Copy"/>
   <MenuItem Command="ApplicationCommands.Paste"/>

<local:ObservableMyObjectCollection>